Towed arrays have been used for some time as listening devices for detecting the presence of underwater sound sources. Such arrays consist of a series of interconnected hydrophones with the requisite electronics encased in a flexible tubular jacket. These arrays may be manufactured in sections of any desired length, such as 50 or 100 feet, which may be connected end to end to produce a much longer array. Such arrays are then towed behind a ship, often at a substantial distance and at moderate speeds to minimize noise related to turbulence from the ship's wake and from velocity effects. So long as the array is being pulled through the water certain longitudinal acceleration and deceleration forces on the array are inevitable, and these forces tend to result in the production of spurious signals from the hydrophones.
The problem of acceleration-induced spurious signals has been dealt with in earlier hydrophone designs by placing pairs of hydrophones physically back to back to produce a structure in which longitudinal accelerations tend to shorten one element while elongating the other, thus canceling or substantially canceling the spurious acceleration-induced signals. Frequently such hydrophones have used hollow cylindrical transducer elements of piezoelectric material having both inside and outside surfaces exposed to oil and having an orifice or port to permit oil to flow across the wall for pressure compensation. The pressure-equalizing port has been found to introduce undesirable phase shifts into the output--at some frequencies, at least. Also, the hollow ceramic elements tend to be fragile and subject to damage from rough handling on deck. Flexing of the side walls of the acoustic elements has also been shown to introduce some spurious signals.
In an effort to overcome some of the above problems, one of the applicants herein and another devised the hydrophone shown and describe in U.S. Pat. No. 4,017,824 (common assignee). The patented design employs solid ceramic pizeoelectric transducer elements affixed back to back against a central bulkhead. The outside end surfaces of the elements were bonded to end caps physically sealed to the inside of the housing with O-ring seals. The volume inside the end caps not occupied by the elements contains air so the hydrophone is not pressure-compensated and must resist the ambient pressure. While this hydrophone is quite operative, it has disadvantages in that the lack of pressure compensation results in an excessive stress in the piezoelectric element at great depths. It is also believed that the non-pressure-compensated design also suffers from a disadvantage in that, on a long term basis, the piezoelectric characteristics of the elements are subject to change where they are wholly or partially unloaded on the side walls. Another problem area is in the difficulty of assembly. The hydrophones described in the patent referred to above (both versions) are also somewhat difficult to assemble.